= Stirling number of the second kind
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{title2=$\left\{\begin{smallmatrix}n\\k\end{smallmatrix}\right\}$}
{wiki=Stirling_numbers_of_the_second_kind}
The Stirling number of the second kind $\left\{\begin{smallmatrix}n\\k\end{smallmatrix}\right\}$ counts the <set partitions> of an $n$-element set into $k$ blocks. It satisfies
$$
\left\{\begin{matrix}n\\k\end{matrix}\right\}
=k\left\{\begin{matrix}n-1\\k\end{matrix}\right\}
+\left\{\begin{matrix}n-1\\k-1\end{matrix}\right\}.
$$
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